1. Field of the Invention
The present invention relates to a method of testing performance or functionality of a pixel circuit device within a display. More particularly, the present invention relates to a method of testing performance of a pixel within a display before forming a lighting device in the pixel.
2. Description of the Related Art
One of the earliest recordings of dynamic images can be found in documentary movies. Later on, when the cathode ray tube (CRT) was invented, television is brought into each family. With the advent of the computer age, CRT is adopted as a display monitor for the desktop computer. Despite its popularity, the radiation gun design always poses some hazard to human health and increases the bulk of each unit. Thus, less bulky and less hazardous alternatives to the CRT are sought.
Flat panel displays are the results of intensive research to reduce size and weight of the display. Many types of displays belong to this category. They include liquid crystal displays (LCDs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), organic light-emitting diodes (OLEDs) and plasma display panels (PDPs).
Organic light-emitting diode display is a type of self-illuminating display also commonly referred to as organic electroluminescence displays (OELs). Major characteristics of an OEL are: DC low voltage driven, high luminance, high efficiency, high contrast value and light. In addition, an OEL is capable of producing a spectrum of colors including the three primary colors red (R), green (G), blue (B) as well as white. Hence, OEL has the greatest potential to become the dominant type on the next generation of flat panel displays. Aside from being thin, light, energy-saving, self-illuminating and having a high resolution, a device fabricated using the OEL technique also has a wide viewing angle, a brilliant color contrast and relatively low cost of production. With these advantages, it has been broadly applied as a LCD or backlight in an indicator panel, a mobile phone, a digital camera and a personal digital assistant (PDA).
According to the driving method, OEL can be classified into passive matrix driven type and active matrix driven type. A passive matrix driven OEL has a rather simple structure and does not require any thin film transistor (TFT) to drive the circuit and hence has a lower production cost. However, the passive matrix OEL has only moderate resolution and poor displaying capacity. Furthermore, as size of the display panel is increased, power consumption is increased and working life is shortened. On the other hand, active matrix OEL technique can be applied to form a large display screen with a wider viewing angle, a higher illumination and a quicker response. The only drawback is that it has a higher production cost than a passive matrix OEL.
OEL display panels are formed generally by first forming (e.g., by a lithography and deposition process) the drive circuits comprising the necessary scan and data lines and electronic devices (e.g., switching transistors), and then forming the organic functional layer (e.g., by a lithography and deposition or coating process). FIG. 1 is a circuit diagram of the driving circuit of a conventional pixel of the OEL display. As shown in FIG. 1, before forming the organic functional layer (the location of the organic functional layer is symbolically represented by the circle 102 in FIG. 1), the pixel circuit 10 includes a scan line 104, a data line 106, a switching transistor 108, a driving transistor 110 and a capacitor 112. Since a source terminal of the driving transistor 110 is in an open circuit condition before forming the organic functional layer, it is impossible to test the performance of the circuit, such as by measuring the charge, voltage or current flowing through the driving transistor 110 via the drain terminal of the driving transistor 110. In other words, all the components inside the pixel circuit 10 can only be tested after forming the organic functional layer in a conventional pixel testing method. This is undesirable because the testing procedure is proceeded after the formation of the organic functional layer and the cathode. Therefore, the yields and the cycle time of the active matrix OEL display panels are difficult to control.